The proposed research program is designed to verify the hypothesis that facet load is transmitted from one vertebra to the next by having the inferior facet bottom out on the pars interarticularis of the vertebra below. It is further hypothesized that the facets rotate relative to each other under load, causing the joint capsule to be stretched. Measurements are proposed to verify this second hypothesis. The specific aims of the research program are: 1. To develop a loading system which can simulate extensor muscle action and apply flexion loads on a motion segment. 2. To measure contact pressure of the facets on the pars interarticularis. 3. To measure motion (translation and rotation) of the facets during application of axial load to the lumbar spine. 4. To use the three-dimensional finite element model of a lumbar motion segment to simulate the motion of the facets due to gravitational and muscular loads. A combined analytical and experimental approach is proposed because the experimental data acquired from cadaveric specimens cannot reflect the effects of muscular action and the mathematical model of a motion segment is an essential surrogate at this stage of the investigation of low back pain. Recent data on facet joint stiffness indicate that muscular tension, especially of the spinal extensors, can play a significant role in the distribution of load borne by the disc and the facets.